This invention relates to the packaging of electronic assemblies and, more particularly, to a construction of such an assembly wherein subassemblies may be easily removed for reworking or repair.
Electrical and electronic assemblies intended for many space and military applications are assembled in a modular form, wherein components are assembled into subassemblies, and then the subassemblies are assembled to form the assembly. To the extent logically possible, the assemblies are assembled from the subassemblies within a single frame, commonly termed a "slice". The slices may in turn form part of a larger, more complex device. By way of example, a small, special purpose computer can be assembled by placing the central processor into one subassembly, a clock into another subassembly, input/output functions into another subassembly, and memory into one or more additional subassemblies. The subassemblies, typically of the size of 1 inch wide by 5 inches long by 0.2 inches thick, are then mounted together in a laterally side-by-side relationship within a frame which is large enough to physically accommodate the subassemblies. The frame usually includes side rails to which the subassemblies are attached, and a header card for use in electrically connecting the subassemblies together as necessary. The frames thus provide a convenient structure for assembling the device, and for handling the device as a single unit.
If such a device fails during operation, the entire frame can be quickly removed by a service technician and replaced with an operating device. The failed assembly contained within the frame can then be inspected to determine which subassembly has failed, and the failed subassembly is replaced. The subassemblies and frames are designed with such procedures in mind, so that circuitry and inspection points are provided to determine the precise location of a failed subassembly or component.
The described construction is widely used in space and military applications, where the electronic assemblies may be subjected to mechanical shocks and vibrations. For example, when a satellite is boosted from earth to orbit, the electronic components may be subjected to considerable vibration, and the electronic components, subassemblies and frames must be packaged in such a way as to withstand the vibrations with no damage. One effective approach for such packaging has been found to be encapsulation of the sensitive subassemblies in a rigid, shock absorbing foam. Encapsulation is accomplished by first inserting the subassemblies into the frame, placing chemicals into the spaces within the frame between the subassemblies, and allowing these chemicals to react to form a foamed reaction product. The foam fills the spaces between the individual components of the subassemblies, between the adjacent subassemblies, and between the subassemblies and the sides of the frame.
Foam encapsulation has been found to be a highly effective way of protecting the devices constructed in the described manner. However, the foam also acts much like a glue or adhesive to bond the subassemblies tightly into the slice assembly. At a later time, if it is necessary to repair the device contained within the slice, the foam makes the removal of a subassembly very difficult and time consuming.
To remove a subassembly, it was long the standard practice that a rework technician would use a sharp tool to manually dig, pick, carve, slice and fracture the foam around the subassembly to isolate it from the remainder of the foam, and then gradually work the subassembly free to allow replacement. The subassemblies are usually laterally spaced less than about 1/2 inch apart within the frame, and this manual procedure often resulted in unintended damage to the subassembly being replaced, and sometimes to adjacent subassemblies. A skilled worker might require at least 1/2 hour to remove the subassembly from its encapsulation, and less skilled workers might require as much as two or more hours to remove the subassembly, a process termed "defoaming".
An important advance in the defoaming of subassemblies is disclosed and claimed in U.S. Pat. No. 4,414,606, which describes a structure and technique for accomplishing a portion of the defoaming process. Specifically, a mechanical cutting medium in the form of a sinuous filament is placed between each pair of laterally adjacent subassemblies, so that the foam between the subassemblies can be fractured by pulling the filaments to removed them from the foam. As the filaments are removed, the foam between adjacent subassemblies is cut.
The disclosure of U.S. Pat. No. 4,414,606 indicates that, once the foam is cut, nuts and electrical connections which would hold the subassembly in place are removed, and the subassembly is then simply lifted out of the frame. However, for some arrangements of the subassemblies and frames, the situation is not this simple, and it may be difficult to lift the subassembly out of the frame even after the foam is cut in the manner described. The difficulties arise because the foam acts like a glue to bind the subassembly to the frame itself, and because geometrical constraints make it impractical to use the mechanical cutting approach between the subassembly and the frame. The subassembly therefore tends to adhere to the frame along the side rails and header board of the frame, and it may be difficult to remove the subassembly. The technician therefore must resort to the prior technique of digging away the foam with a sharp instrument, but may then damage the components or expand large amounts of time in the removal process.
Thus, while U.S. Pat. No. 4,414,606 represents an important advance in the art of defoaming, there remains a further need for an assembly which is adapted for removal of subassemblies for repair or reworking, and a method by which subassemblies may be removed without damaging the removed subassembly or adjacent subassemblies. Such as assembly and method must be fully compatible with the electronic devices themselves, since it is not possible to redesign the subassemblies, frames, or device electronics to accommodate the removal and reworking needs. The present invention fulfills this need, and further provides related advantages.